Never offer help unless you're willing to give it

I, being the nice gf that I am offered the bf help tearing down his deck on Sunday. I've been offerring to help for a few weeks, and every time he has turned me down, thank god. Except for this last time, I offerred expecting for him to say no, but to my surprise he said "yeah I think I do need it." Because of his awesome planning, and busy schedule, he left it to the day before it needed to be down so that the roofers could re-roof their building. So, for about 4hrs I made about a million trips up and down two flights of stairs, carrying pieces of the deck to the dumpster. All the while I was thinking, I really need to go sleep (because I was working a 12 hour shift that night from 7:30pm-8am. Now every time I walk, my calves scream in pain from all the stair climbing on Sunday. Moral of the story, don't offer help unless you really are up for the task....